Reverberation device

ABSTRACT

A reverberation device including a frame having a reverberating metal foil of very small thickness. The metal foil comprises a nearly pure metal having the characteristics of high thermal conductivity ( lambda ) &gt;/= 69 W/m DEG , a low coefficient of thermal expansion ( alpha ) &lt;/= 19.0 x 10&lt;-&gt;6/ DEG , a low sound velocity c = (E/ rho )1/2 &lt;/= 5,700 m/s wherein the square of the coefficient of thermal expansion ( alpha ) of the foil, when multiplied by the square of the velocity of sound c and divided by the thermal conductivity ( lambda ) is less than 1.3 x 10&lt;-&gt;5 m3/ DEG S2W.

United States Patent [191 Kuhl et al.

1 1 March 6, 1973 1541 REVERBERATION DEVICE [75] Inventors: Walter Karl Kuhl; Jens Wieklng,

'both of Hamburg, Germany 73 Assigneez' Elektromerstechnlk Wilhelm Franz KG,Labor/Schwarzwald, Germany 221 Filedi 'April27, 1971 21 Appl.No.i 137,840

30 Foreign Application Priority Data April-29, 1970 Germany ..1 20 20 897.3

[52] Us. (:1.- ..;..;......333/30 R, 179/1 J, 84/124, 84/DIG. 26 [51] Int. CI.....L ..H03h 9/30 [58] Field of Search ....333/30 R; 179/1 J; 84/124, 84/DIG. 2'6

[56] References Cited UNITED STATES PATENTS 2/1960 Kuhl ..179/1 .1

. FOREIGN PATENTS OR APPLICATIONS 1,472,007 11/1968 Germany ..179/l J 1,045,117 11/1958 Germany ...l79/I .1 1,073,215 1/1960 Germany ..l79/l .1

OTHER PUBLICATIONS Reference Data for Radio Engineers Fifth Edition H. W. Sams & Co. 1969, two pages: Title page and 4-45.

Primary Examiner-Herman Karl Saalbach Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT A reverberation device including a frame having a reverberating metal foil of very small thickness. The

metal foil comprises a nearly pure metal having the characteristics of high thermal conductivity (A) :69 W/m, a low coefficient of thermal expansion (01) 19.0X10- a low sound ye1oc ity c E/p) sITbHi/E vvherein the square of the coef fi cien tjf thermal expansion (01) of the foil, when multiplied by the square of the velocity of sound c and divided by the thermal conductivity (it) is less than 1.3

10 Claims, 9 Drawing Figures PATENIEUMAR 6|973 ,719,905

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, 1 REVERBERATION nnvrcn For special acoustic application such as the genera-' tion of artificial reverberations, thin and absolutely plane metal plates are'required with a length or width of the order of magnitude 10,000-fold to that of their thickness.

Known reverberation devices comprise a steel plate 1% mm thick and l 'k-Z m in area as the reverberating body. The size and the weight of the device are cumbersome. If one wishes to reduce its dimensions, then thickness must be reduced to the same extent as area. Sound improvement of reverberation even requires reducing the plate thickness more than the area. However because of the reduction in thickness and when dealing with iron, nickel, aluminum and many other metals, the plates damping factors may so increase that it is impossible to achieve therequired reverberation times of 4-5 seconds at low and middle frequencies and of at least 0.8 seconds for a frequency of 10 kHz with respect to mechanical recordings;

However, if certain material property limits are observed, it is possible to make a thin plate or foil from metals with such properties and in such manner that approximately the indicated minimum reverberation times are achieved, or even exceeded. Three decisive material properties must always be observed within the following limits: heat conductivity A Z 69 watt/m.deg.; thermal expansion coefficienta 19 X ldeg.; sound velocity c=(E/p S 5,700 m/sec. However, there are metals which have these properties but that at low or high frequencies show'too much damping for any foil thickness, for optimum properties, which require still another condition. The square of the thermal expansion coefficient alpha of the foil material or materials multiplied by the square of sound velocity c and divided by the heat conductivity lambda must be less than 1.3 X mldegsec. watt. Metals meeting these conditions are gold, silver, copper, platinum, chromium, molybdenum and tungsten, particularly the first four.

Very thin and very thick foils show too much damping at the upper end of the audio frequency range. Therefore, the foils according to the invention were made 0.008 -0.035 mm thick, preferably 0.01-0.03mm depending on the metal. Optimum thickness is 0.02 mm.

Such thin foils are appreciably more sensitive to acoustic interferences, particularly as transmitted through air and solids, than are the known reverberation plates of 0.5 mm thickness. Hence a two-shell housing is used in this invention. Since the required airtransmitted sound insulation is'practically independent of frequency, or very nearly so, and therefore must be considerable even at low frequencies, one of the two housings is curved'in three dimensions and very stiff shells are formed. The second housing may consist of thin, flat or curved sheet-metal since its transmission loss need only be largefor middle frequencies at which the first housingresonates and therefore has less of a transmission loss.

The required and very large solid transmitted sound insulation is achieved by first, isolating the foil from its supporting frame,

second, isolating the frame from the inner box and third, isolating the inner from the outer box byvmeans of damped springs. If required, the outer box too will be spring-isolated from the supporting founda tion.

Parts of the air volume of the inner and outer boxes are filled with sound absorbing material with open pores, preferably mineral fiber plates, foam rubber or foam plastic. This porous material may not be mounted close to the plate itself, inside the inner box, so as not to unduly increase the plates damping. Parts of the porous material simultaneously may serve as the frame spring-support.

The reverberation time for thick reverberation plates may be changed in known manner by nearing or removing porous layers of large areas. For middle and small values of reverberation the time of reverberation is fairly frequency-independent. For thin reverberation foils therefore the low frequency reverberation time would be much less than for middle frequencies. This is avoided according to the invention by the porous layer consisting of several small parts in one plane which differ with respect-to their sizes and spacings from one another. The porous layer may also be divided by means of numerous perforations of equal or varying sizes into several mechanically interconnected partial areas.

The accompanying drawings show diagrammatically an embodiment and details of the echo plate in accordance with the invention.

FIG. 1 shows a view of the echo plate without a box.

FIG. 2 shows a section through the plate and a double-walled housing.

FIGS. 3 and 4 show embodiments of the weight.

FIGS. 5 and 6 show embodiments of the damping or reduction plates.

FIGS. 7, 8, and 9 show embodiments and variations 7 of the echo time owing to the use of eddy current rigidly with the remaining parts. The number of con- I nection points depends how the foil has been produced, how flat it must be in order to avoid audible oscillation distortions and how strong the tension force is to be for reducing foil damping or reduction. Instead of the ten sioning in the direction of the foil edges by means of two springs adjacent to each corner the foil can be tensioned at the comers by one respective spring in a diagonal direction.

The oscillation excitation and sensing is carried out in the embodiment shown electrodynamically at two points with the help of very light oscillating coils 7 connected rigidly with the foil. The associated permanent magnets, in whose air gaps the coils are located, are not shown for purposes of simplification. For monophonic operation one oscillating coil would suffice for exciting and sensing. in the case of two channel stereophonic operation at least two sensing oscillating coils must be provided. Two exciting oscillating coils instead of only one are then in this case acoustically more advantageous. in the case of more than two stereochannels the number of the oscillating coils for sensing is increased. As far as possible the number of coils for excitation is also increased. The coils are arranged at a distance from each other and from the edges. Their positions can in other respects be chosen freely.

The echo time is increased by the distance of the porous plate 8, provided in this case with slots or holes, of the oscillating foil (plate) is changed by hand or by means of a remote control system.

The inner box 9 consists of two curved metal shells,

' and the outer box 10 in the embodiment shown consists of flat plates, which are produced using any suitable material. The partial filling of the air spaces in the two boxes with porous sound absorbing material is not shown in order not to impair understanding of the drawing. The frame 2 is connected with the inner box 9 by means of springs 11. The box 9 is connected with the outer box 10 by means of springs 12. Between the outer box 10 and the supporting ground there are springs 13.

FIGS. 3 and 4 show a view of two embodiments of the weights 3 which consist of round discs 14 and 15 pressed together by means of a screw 16. Each disc has two pins 17, between which the foil 1 is clamped. An additional connection by means of putty with low damping properties may be convenient. The spring ring 4 in FIG. 3 is placed around the pins 17 and led through the second spring ring 5. An additional point-like rigid fixation of the spring rings on all points of connection can also be convenient in this case. FIG. 4 shows the U- shaped wire spring 18 which is connected rigidly with the pins 17 and with the holding part 6. A displacement of the holding part 6 before it is screwed into position can be used to set the desired tension.

The damping plate in FIG. 5 consists as an example of alarge quantity of parallel strips 19 of porous material with open pores. The breadth of the strips is variable, and also that of the intermediate spaces 20.

FIG. 6 shows a further embodiment. The porous plate 21 has regular or unregularly arranged large openings 22 and small openings 23.

FIGS. 7 to 9 show embodiments of variations of the echo time with the help of eddy current damping. FIG. 7 shows a few permanent magnets 24 with rectangular or circular cross-section placed on both sides of the foil 1. The polarity of the magnets placed opposite to one another is the same and those of adjacent magnets is different. Thus, a strong magnetic flux is achieved in the plane of the foil. The eddy current damping of the foil is achieved by displacing the distance of all magnets from the foil.

FIG. 8 shows a pot magnet 25 whose leak flux partially flows through the foil 1. Several magnets on the same side of the foil and oppositely poled magnets on the other side can increase the damping or reduction.

FIG. 9 shows several electromagnets 26 with exciting spools 27 on one side of the foil 1. The magnetic field and thus the damping is varied by the variation of the distance and/or of the exciting DC.

We claim:

1. A reverberation device comprising a frame within which is stretched a thin, reverberating metal foil, said foil comprising at least one layer of nearly pure metal having a large heat conductivity A g 69 wattlmdeg. and a small thermal expansion coefficient a 19.0 X l0'/deg. and a small sound velocity c=(E/p) 5 ,700m/sec. wherein the square of the thermal expansion coefficient alpha of the fOll when multiplied by the square of the sound velocity 0 and divided by the heat conductivity lambda is smaller than 1.3 X 10' m /deg. sec. 2. Watt and wherein the metal foil thickness is between 0.008 mm and 0.035 mm.

2. A reverberation device according to claim 1, wherein the metal foil thickness is within the range of 0.01-0.03 mm.

3. A reverberation device according to claim 2, wherein the foil metal make-up is at least 80% pure of the material selected from the group consisting of gold, silver, copper, platinum, chromium, molybdenum or tungsten.

4. A reverberation device according to claim 1, wherein the reverberating foil is mounted inside a first box closed on all sides and wherein this box is made of material with a large modulus of elasticity, and comprises at least two partial shells that are bent in three dimensions.

5. A reverberation device according to claim 4, wherein a second outer box is provided for increasing the transmission loss and which integrates the inner box comprising at least two shells.

6. A reverberation device according to claim 4, wherein the metal foil, the frame used for stretching the metal foil, the first box and the second box and the foundation all are oscillation isolated from each other by damped springs.

7. A reverberating device according to claim 5, wherein the inner and outer boxes are partly filled with an open-pore sound absorbing material.

8. A reverberating device according to claim 7,

wherein the sound-absorbing material in the inner and 

1. A reverberation device comprising a frame within which is stretched a thin, reverberating metal foil, said foil comprising at least one layer of nearly pure metal having a large heat conductivity lambda > or = 69 watt/m.deg. and a small thermal expansion coefficient Alpha < or = 19.0 X 10 6/deg. and a small sound velocity c (E/ Rho ) < or = 5,700m/sec. wherein the square of the thermal expansion coefficient alpha of the foil when multiplied by the square of the sound velocity c and divided by the heat conductivity lambda is smaller than 1.3 X 10 5 m3/deg.sec2.watt and wherein the metal foil thickness is between 0.008 mm and 0.035 mm.
 2. A reverberation device according to claim 1, wherein the metal foil thickness is within the range of 0.01-0.03 mm.
 3. A reverberation device according to claim 2, wherein the foil metal make-up is at least 80% pure of the materiAl selected from the group consisting of gold, silver, copper, platinum, chromium, molybdenum or tungsten.
 4. A reverberation device according to claim 1, wherein the reverberating foil is mounted inside a first box closed on all sides and wherein this box is made of material with a large modulus of elasticity, and comprises at least two partial shells that are bent in three dimensions.
 5. A reverberation device according to claim 4, wherein a second outer box is provided for increasing the transmission loss and which integrates the inner box comprising at least two shells.
 6. A reverberation device according to claim 4, wherein the metal foil, the frame used for stretching the metal foil, the first box and the second box and the foundation all are oscillation isolated from each other by damped springs.
 7. A reverberating device according to claim 5, wherein the inner and outer boxes are partly filled with an open-pore sound absorbing material.
 8. A reverberating device according to claim 7, wherein the sound-absorbing material in the inner and outer boxes at the same time serves as a spring-support.
 9. A reverberating device according to claim 1, wherein a porous layer of several small component parts in one plane with varying but small distances from the foil are assembled for the purpose of decreasing the reverberation time of the reverberating foil. 